Omit empty pool proxy_id

buildPoolConfigParams included proxy_id whenever it was defined, so an
empty string was forwarded to the API on pool create/update. Match the
truthiness check manage_browsers create uses (and the sibling name
field) so empty proxy_id is dropped; pools have no clear-proxy semantics
for an empty string.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: IlyaasK

src/lib/mcp/tools/browser-pools.ts

@@ -59,7 +59,7 @@ function buildPoolConfigParams(
       ...(params.timeout_seconds !== undefined && {
         timeout_seconds: params.timeout_seconds,
       }),
-      ...(params.proxy_id !== undefined && { proxy_id: params.proxy_id }),
+      ...(params.proxy_id && { proxy_id: params.proxy_id }),
       ...(params.fill_rate_per_minute !== undefined && {
         fill_rate_per_minute: params.fill_rate_per_minute,
       }),